I remember back in the 50's, yes I'm that old, when people, mostly in gangs, use to make guns of very simple materials. They were called "Zipguns". Maybe some of the "older" guys and gals will remember that.
all you needed was a base, a barrell, and some kind of mechanizim to triger the firing pin, and you had a "Zipgun". They were also popular in prisons, or so I'm told, ya understand? Saw it on the TV, don't ya know?